Enhanced structural and cycling stability of O3-type NaNi1/3Mn1/3Fe1/3O2 cathode by Al replacement of Mn studied in half cell/full sodium-ion batteries

Abstract

This study uses ball-milling spray drying to prepare a hollow spherical cathode material by doping non-electrochemically active Al into O3-NaNi1/3Mn1/3Fe1/3O2 (NMF) cathode material. The effects of modification on the material's properties were analysed, and the optimum-doped ratio of NaNi1/3Mn(1/3−x)Fe1/3AlxO2 (x = 0.005, NMFA 0.005) was determined. For half-batteries, NMFA 0.005 exhibits a good initial discharge capacity of 115.1 mAh g⁻1 at 1 C and capacity retention of 87.8 % after 200 cycles, which is 16.1 % higher than that of NMF samples. Furthermore, at a high rate (5 C), the initial capacity was 92.3 mAh g−1 and the capacity decline rate was 16.14 %. However, the initial discharge capacity at high voltage was still 136.9 mAh g⁻1. For full batteries, NMFA 0.005 delivered a discharge capacity of 81.1 mAh g⁻1 with a capacity retention of 82.2 % after 100 cycles. Capacity retention was reinforced by approximately 37 % compared with the matrix. Ex situ X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) failure analysis results put together show that an appropriate amount of Al doping could successfully alleviate the Jahn–Teller effect caused by Mn3+, decrease the oxygen loss and increase the stability of the layered structure, all of which result in an excellent electrochemical performance.